So, i\'ve been having this problem for a while now.
Notepad++ NppFTP plugin fails to connect to server via SFTP with \"[SFTP] Error during authentication: Invalid p
When using Notepadd ++ I found that if your logging into a secure server with NppFTP and you know your user name and password but your getting [SFTP]Unable to authenticate error: Try going to toolbar -> Profile settings -> Authentication and only check the box that says try password authentication. The default is private file but, that doesn't all ways apply.
-Hope that helps-